Modelling water-harvesting systems in a semi arid catchment (Merguellil-Tunisia)

In the Mediterranean regions, hydrologic processes are quite specific due to the temporal variability of precipitation characterized by a succession of drought and flash-flood periods. These processes may also have changed due to a range of human activities such as land use changes, dams building, soil and water conservations works. The Merguellil catchment (Central Tunisia) is a typical Mediterranean semi-arid basin which suffers regular water shortage aggravated by current drought. During the recent decades the continuous construction of small and large dams and Soil and Water Conservation Works (SWCW) (ie. Counter ridges) has taken place within the watershed. However, little is known about the effect of these water harvesting systems on the water balance components of arid watersheds. The work presented here attempts to simulate the actual water balance using the Soil and Water Assessment Tool (SWAT-2005) model including the water harvesting systems. Large dams are modeled as reservoirs, small dams as ponds, and contour ridges as potholes that fill with water, and increase the percolation into the aquifer. The model was rather successful in reproducing water flow. Two scenarios were further generated. The first one regards the removal of contour ridges to assess their impact to water and sediment load. The second scenario consists in planting the olive trees between contours. The results of the first scenario show that the contour ridges contribute to the retention of high quantity of sediment. These regulations reduce the surface runoff by 32 %.The second scenario shows that planting the olive trees between contours could improve its yield.